Problem with UV mapping (single face dis formed)
Use a different operator with N-gons

N-gons are very handy, but they come at the cost of not having a precisely defined topology between the vertices. Blender is struggling to recognize the shape of the surface, the proportion of each branch, with the Unwrap operator due to the algorithm that's behind it.

I'll suggest to simply use another operator for this specific task, for example the Smart UV Project operator: